Create cap_anndata_schema.md

[mfutey]

• Added missing sections, e.g. [cellannotation_setname]--category_fullname, [cellannotation_setname]--category_cell_ontology_exists, etc
• Added "source," "required for publication on CAP" and "example" fields and values for each section
• Checked for typos and consistency
• Changed format from bullet points to tables
• Removed character limit for descriptions
• Added 200 character limit for titles
• Corrected .var text to indicate ENSEMBL terms are required not gene names
• Removed "programmy" language, e.g. ndarray